I'm having the same problem, but "MOZ_DISABLE_PANGO=1 thunderbird" is only
fixing it for a local copy of thunderbird. I need to get my corporate email via
an X-remote session over ssh and thunderbird on the remote system still doesn't
work when started like this. Both ends are uptodate Fedora Rawhide.

I suspect that the new /usr/bin/sort from coreutils-5.92-1.x86_64.rpm is
breaking something in fontconfig's font caching.
I am able to fix the thunderbird problem by:
downgrading to coreutils-5.2.1-56
reinstalling fontconfig-2.3.91 to rebuild its caches using "rpm -Uvh
--replacepkgs"
See also bug#172028 "sort breaks autofs"

I was able to fix the problem (and run firefox and thunderbird again) by just
running fc-cache -f -s as root which is the post-install script for fontconfig.
I don't think this one has anything to do with sort; reinstalling fontconfig is
what made it work for John Ellson.